Apparatus for the measurement of ultra-violet, visible and/or infra-red radiation

ABSTRACT

A radiation measuring device for ultra-violet, visible or infrared radiation, particularly suitable for use in a color analyser for photographic printing has a photomultiplier tube with a current sensing resistor in the anode circuit. The voltage across this resistor is used to control the amplitude of oscillation of an oscillator which, via a step-up transformer and rectifier circuit, provides E.H.T. for the tube. The E.H.T. voltage is thus controlled to maintain the anode current sensibly constant. The controlled E.H.T. voltage also is applied to the bleeder resistor chain for the dynodes of the photomultiplier tube, which resistor chain, with a series measuring resistor, forms a potential divider; the voltage across the measuring resistor is proportional to the logarithm of the light intensity and a temperature compensated correction circuit is provided to improve the linearity of this relationship. The voltage across the measuring resistor is applied to one input of a differential voltage measuring circuit having, in the case of a color analyzer, a second input from potential sources switched in synchronism with the color filter selection.

United States Patent Davies [451 Apr.4,19'72 [54] APPARATUS FOR THEMEASUREMENT OF ULTRA-VIOLET, VISIBLE AND/0R INFRA-RED RADIATION Appl.No.: 34,344

Primary Examiner-Ronald L. Wibert Assistant Examiner-V. P. McGrawAttorney-Mawhinney and Mawhinney [57] ABSTRACT A radiation measuringdevice for ultra-violet, visible or infrared radiation, particularlysuitable for use in a color analyser for photographic printing has aphotomultiplier tube with a current sensing resistor in the anodecircuit. The voltage across this resistor is used to control theamplitude of oscilla- Foreign Application Priority Data tion of anoscillator which, via a step-up transformer and recti- I fier circuit,provides E.H.T. for the tube. The E.l-l.T. voltage is y 5, 1969 GreatBmal" -22882/69 thus controlled to maintain the anode current sensiblyconstant. The controlled E.H.T. voltage also is applied to the [52] US.Cl ..356/51, 250/207, 356/74, bleeder resistor chain for the dynodes ofthe photomultiplier 356/186 356/202 tube, which resistor chain, with aseries measuring resistor, [51] Int. Cl ..G0ln 21/34, G01 3/00, GOln21/0o forms a potential dividerm.1e voltage across the measuring FieldOf Search 1 sister is proportional to the logarithm of the intensity and250/207 02 a temperature compensated correction circuit is provided toimprove the linearity of this relationship. The voltage across [56]References Cited the measuring resistor is applied to one input of adifferential UNITED STATES PATENTS voltage measuring circuit having, inthe case Of a color 1 analyzer, a second input from potential sourcesswitched in 2,971,433 2/1961 Akin ..250/207 synchronism with the colorfilt Selection 3,130,316 4/1964 Townsend. .....250/207 I 3,528,749 9/1970 Bowker ..356/202 11 Claims, 4 Drawing Figures F'Fjv 3O 46\ ISTABILIZED 34 69 PUWER 76 64 SUPPLV L 63 .l. .2 68 47 F vi ,vv T 48 6/iii 66 67 :E

APPARATUS FOR THE MEASUREMENT OF ULTRA- VIOLET, VISIBLE AND/OR INFRA-REDRADIATION BACKGROUND OF THE INVENTION 1. Field of the Invention.

This invention relates to apparatus for the measurement of ultra-violet,visible and/or infra-red radiation.

2. Description of the Prior Art.

It is often required to measure the intensity of radiation, e.g., inspectrophotometers, densitometers, color analysers, exposure meters andscintillation counters. For many purposes, results are expressed interms related to optical density which is the logarithm of thereciprocal of the transmissivity and for this reason it is desirable tohave an output display with a linear scale representative of thelogarithm of the measured radiation. Use commonly has been made of aphotomultiplier tube as the light sensing device, the photomultipliertube having a chain of bleeder resistors connected to the variousdynodes which chain, with a further resistor is connected in series witha control tube, usually a pentode tube. The grid of the control tube isconnected to the anode of the photomultiplier tube. A constant E.H.T.voltage is applied between the anode of the control tube and the cathodeof the photomultiplier tube. With such an arrangement, as the lightintensity increases, the anode current of the photomultiplier tube tendsto increase, so reducing the grid potential of the control tube. Thecurrent through the bleeder resistors is reduced so that dynode voltageand thereby the sensitivity of the tube is reduced. With a high gaincontrol tube, the anode current is maintained substantially constant.With a typical photomultiplier tube, the dynode voltage in such anarrangement is, to a reasonably close approximation, linearlyproportional to the logarithm of the sensitivity.

The photomultiplier tube will typically require a voltage of over 1 kv.and the practice has been to control the current through thephotomultiplier tube by means of a control tube with a series resistor.One such arrangement is described by Sweet in Electronics Nov. 1946pages 105-109 and a number of variations and developments of thisequipment have been made since then. However such equipment is heavy andbulky and has a high power consumption. Moreover, control tubes such asare used in this equipment have a limited life. It is not possiblehowever to use a transistor or other three-terminal semiconductor deviceinstead of the control tube as a constant current supply source for thephotomultiplier tube because of the very high potential for thephotomultiplier tube.

SUMMARY OF THE. INVENTION It is an object of the present invention toprovide an improved form of radiation sensing device responsive toultraviolet, visible or infra-red radiation making use of aphotomultiplier tube to give an outward indication approximatelylinearly proportional to logarithm of the radiation intensity and which,apart from the photomultiplier tube, can employ solid state devices.

According to this invention a radiation sensing device responsive toultra-violet, visible or infra-red radiation comprises a photomultipliertube having an anode, a cathode and dynodes with a bleeder resistorchain across the dynodes, an E.H.T. supply means applying an E.H.T.voltage between the anode and cathode and also across the bleederresistor chain, said E.H.T. supply means including a step-up transformerand a rectifier, an oscillator unit applying alternating current to thetransformer, means responsive to the anode current of thephotomultiplier tube arranged to control the amplitude of the output ofthe oscillator unit so that the E.H.T. voltage is decreased as the anodecurrent tends to increase so as to maintain the anode currentsubstantially constant and indicator means responsive to the E.H.T.voltage. By this arrangement using an oscillator and a step-uptransformer so that the overall E.H.T. voltage is varied to keep theanode current substantially constant, the oscillator unit can be a lowvoltage device which may readily be constructed using three-terminalsemi-conductor devices such as transistors. The oscillator unit mayinclude an oscillator of any suitable type, for example a Hartley or aColpitts circuit may be used or a multivibrator circuit. The oscillatormay feed the transformer directly or through a buffer amplifier and/orvia power amplifier transistors. A single output oscillator may beemployed or a push-pull arrangement.

The aforementioned means responsive to the anode current of themultiplier tube may comprise a resistor in series with the anode and anamplifier responsive to the voltage across said resistor to provide adc. supply for the oscillator unit whereby the oscillator unit outputvoltage depends on the amplifier input. This amplifier may for examplebe a Darlington pair or multiple or may be an operational amplifierconstructed as a discrete component unit or as a thick film ormonolithic unit.

The step-up transformer and rectifier may be a simple half waverectifier system employing a single rectifier and a smoothing condenserbut more complex arrangements, including voltage doublers or multipliersmay be employed.

The output indication may be obtained by measuring the current throughthe bleeder resistor chain; this current is proportional to the E.H.T.(and to the dynode voltage). For this purpose, a current measuringdevice may be connected in series with the bleeder resistor chain at thelow voltage end thereof. As will be explained later, however, it isdesirable to be able to set the datum for the indication to anadjustable level and it is preferred therefore to use a voltagemeasuring device which can be connected to measure the voltage to anadjustable datum level. The anode itself may be at or near earthpotential and the cathode at a large negative potential. Because of thehigh potential, this voltage is preferably measured using a potentialdivider circuit with the meter connected to measure the voltage across aresistor at or near the end of this circuit which is at a low potentialwith respect to earth. The bleeder resistor chain is conveniently usedas part .bleeder resistor chain.

Preferably there are provided two or more rectifiers, each with a seriesresistor, connected to separate adjustable potential sources, eachrectifier being arranged to by-pass part of the current through saidmeasuring resistor in the potential divider circuit when the potentialacross the measuring resistor reaches that of the adjustable potentialsource to improve the linearity of the relationship between thelogarithm of intensity and the indicated voltage, and there is provideda further rectifier in said potential divider circuit between saidmeasuring resistor and the by-pass circuits. The provision of thefurther rectifier compensates for the changes in indication due to thetemperature effects of the rectifiers in the by-pass circuits.

It is preferred to employ a differential voltage measuring device havingone input to which is applied the voltage across the aforementionedmeasuring resistor in the potential divider circuit and a second inputfrom an adjustable potential divider. This potential divider may then beset so that the indicator reads zero at the maximum light intensity orto preset the meter to some arbitrary reference reading.

The radiation sensing device described above finds particularapplication in a color analyser. Such a device may have a number ofseparate light filters which may be switchably placed in the path of thelight radiation reaching the photomultiplier tube and, in such anarrangement, there may be provided a plurality of adjustable potentialdividers which can be switchably connected to the second input of theindicator by a switch synchronised with the switching of the lightfilters thus providing appropriate zeros to be used which have beenpreset for the different colors.

The invention furthermore includes within its scope a densitometerincluding a radiation sensing device as described above in combinationwith a light source. Such a densitometer finds use, for example, inmeasuring the optical density of a photographic negative.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERREDEMBODIMENT Referring to FIG. 1 there is shown a photomultiplier tubehaving an anode l1, cathode 12 and a plurality of dynodes 13 connectedby bleed resistors 14. In series with the anode is a resistor 15, forsensing the anode current, connected to a low voltage supply line 16which, as hereinafter explained, forms one side of the E.H.T. supply forthe photomultiplier tube 10. When this tube draws anode current throughthe resistor 15, a voltage is developed across this resistor dependenton the amount of the anode'current and hence dependent on the incidentradiation. This voltage is amplified in an amplifier 17, the output ofwhich controls the amplitude of oscillation of an oscillator unit 18.Conveniently, this is done by utilising the output voltage from theamplifier'l7 as the supply voltage for the oscillator unit. Thearrangement is such that the output of the oscillator unit increaseswith decrease of radiation received by the photomultiplier tube 10. Thealternating output of the oscillator is applied to a primary winding 19of a step-up transformer 20 having a secondary winding 21. Thissecondary winding is connected in a half-wave rectifier circuit having arectifier 22 and a filter capacitor 23 to provide a smoothed E.I-I.T.supply for the photomultiplier tube 10. In this particular arrangement,the E.I-I.T. supply is a negative voltage with respect to ground and isapplied to the cathode 12 of the photomultiplier tube 10. This cathode12 is connected via a resistor 24 to the chain of bleed resistors 14across the dynodes. The dynode voltage per stage is that proportional tothe applied E.H.T. voltage and is measured by an indicator 25 which maybe a current measuring device in series with the bleed resistor chainbut preferably comprises a measuring resistor in series with this bleedresistor chain and a voltage measuring device responsive to the voltageacross the output of the multivibrator is applied to a transistor 42forming a buffer amplifier feeding a power output transistor 43. Theoutput from this transistor 43 is applied to the primary I9 winding ofthe step-up transformer 20. In FIG. 2, a voltage doubler circuit isemployed. One end of the secondary winding 21 of the transformer 20 isconnected to a tap in a rectifier chain formed by four rectifiers 45,46, 47 and 48 connected between the line 32 and the cathode 12 of thephotomultiplier tube 10. These rectifiers are shunted respectively byresistors 49, 50, 51 and 52. The second end of the secondary winding 21of the transformer is connected to the junction between two smoothingcapacitors 53, 54, the second pole of capacitor 53 being connected tothe cathode end of the aforementioned rectifier chain 45-48 and thesecond pole of capacitor 54 being connected to the line 32.

The amplifier 17 provides an impedance matching betwee'n the anoderesistor of the photomultiplier tube and the control terminal of theoscillator unit 18. The anode load resistor 35 has a magnitude of theorder of kilo ohms to megohms, depending on the requirements of thephotomultiplier tube, whereas the control terminal of the oscillatorunit 18 represents a very much lower impedance. Although a single endedoscillator with buffer amplifier and power output is shown, in somecases it may be preferred to use a push-pull configuration in order tosimplify the design of the step-up measuring resistor. The dynodevoltage per stage of a photomultiplier tube (which in the present caseis proportional to the E.I-I.T. voltage applied between the anode andcathode) will change with changes in incident radiation, the changebeing nearly uniformly proportional to changes in the logarithm of thelight intensity. There is some non-linearity and, as will be describedwith reference to FIG. 2, a correction circuit may be provided tocorrect for the non-linearity in this response.

The apparatus is conveniently constructed with the photomultiplier tubein a probe unit connected by a flexible connector to the remainder ofthe apparatus.

FIG. 2 illustrates in further detail the circuit arrangement of theradiation sensing device of FIG. 1. 5

Referring to FIG, 2 there is shown a stabilised power supply 30providing a low voltage stabilised d.c. supply between an earth line 31and a positive supply line 32. The photomultiplier tube 10 has its anodeconnected via resistors 34 and 35 to the aforementioned line 32. Thepotential across the resistor 35 is applied to the amplifier 17 which isformed by three transistors 36, 37 and 38 in a triple Darlingtonconfiguration.- The dc. output voltage from this amplifier 17 isemployed as a supply voltage for the oscillator unit 18 which is formedby two transistors 40, 41 arranged in a multivibrator circuit. The

transformer 20 or to reduce the regulation requirements of the powersupply unit. Although in the circuit illustrated in FIG. 1, thetransformer is shown with a half-wave rectifier system and FIG. 2 showsa voltage doubler circuit, other arrangements may be employed, forexample a voltage multiplier circuit giving a higher multiplicationfactor.

The cathode 12 of the photomultiplier tube 10 is at a high negativepotential with respect to earth and is connected via resistor 23 to oneend of the chain of bleeder resistors 14 across the dynodes of thephotomultiplier tube. The other end of this chain is connected via aZener diode 60 or a resistor, a diode 61 and a measuring resistor 62 tothe aforementioned line 32. A portion of the total E.H.T. voltage acrossthe photomultiplier tube 10 thus appears across the resistor 62 and thisis fed to the base of a transistor 63 forming one of a pair oftransistors 63, 64 (or a plurality thereof) with a measuring indicator65 connected between the emitters of these two transistors to constitutea differential voltage measuring circuit. The indicator may be ananalogue or digital indicator. The collectors of the two transistors 63,64 are connected to line 32 and the emitters are connected via resistors66, 67 to the earth line 31. The base of the transistor 64 is connectedvia a resistor 68 to a tap on an adjustable potentiometer 69 connectedbetween the lines 31 and 32. This adjustable potentiometer 69 enablesthe meter 65 to be set to zero for example at maximum light encounteredor alternatively enables the meter to be preset to any arbitraryreference reading.

In order to improve the linearity of the relationship between thelogarithm of light intensity and the meter reading, there is provided alinearising or function generator circuit, consisting of a number of setpoint potentiometers of which three are shown at 70, 71 and 72. The tapon potentiometer is connected via a diode 73 and adjustable resistor 74to the junction between the aforementioned Zener diode 60 and therectifier 61. Similarly the tap on the potentiometer 71 is connected viaa diode 75 and adjustable resistor 76 to this point and the tap onpotentiometer 72 is connected via a diode 77 and adjustable resistor 78in this point. These diodes 73, 75 and 77 and adjustable resistors 74,76, 78 provide by-pass circuits which become conductive when thepotential across the measuring resistor 62 and rectifier 61 reachpredetermined values set by the potentiometers 70, 71, 72 and, whenconductive, by-pass predetermined amounts of current from the measuringresistor 62. The potentiometers 70, 71, 72 and resistors 74, 76 and 78are preset to appropriate values to improve the linearity of therelation between the logarithm of the incident radiation intensity andthe changes of E.H.T. voltage as measured on the differential metercircuit. The setting of these potentiometers 70, 71, 72 and adjustableresistors 74, 76, 78 is conveniently effected empirically with acalibration procedure. Whilst only three such by-pass circuits have beenshown, more can be usefully employed and enable greater accuracy to beachieved.

The above-described radiation sensing device may be used in combinationwith a light source as a densitometer, e.g., for measuring the opticaldensity of a photographic negative. The light source conveniently drawsits power from the stabilized power supply source. Such a device isillustrated in FIG. 4 in which a stabilizer power supply source 90 feedsa light source 91 and the sensing device 92 which incorporates aphotomultiplier tube 10. For use as a color analyzer, it may be providedwith a set of filters 94 which are selectively movable into the path ofthe radiation incident upon the photomultiplier 10. For each differentcolor filter, if the readings on the meter 65 (FIG. 2) are to be useddirectly, e.g., for determining processing times, then different off-setvalues will be required to allow for the standing current through thephotomultiplier .tube and, as shown in FIG. 3, it is convenient toprovide a plurality of adjustable current off-sets (corresponding topotentiometer 69 of FIG. 2) which are automatically switched intocircuit in synchronism with the selective insertion of color filtersinto the radiation path.

Referring to FIG. 3, there are shown the two transistors 63, 64 andmeter 65. The voltage to be measured is applied to the base oftransistor 63 by circuit means as shown in FIG.2, only the measuringresistor 62 being shown in FIG. 3. The base of transistor 64 isconnected by a selector switch 80 to a tap on any selected one of threeadjustable potentiometers 81, 82 and 83. These three potentiometers, atone end, are connected in common via an adjustable resistor 84 to thepositive supply line 32. At their other ends the three potentiometersare connected in common to the collector of a transistor 85, the emitterof which is connected via a resistor 86 to the earth line 31. The baseof transistor 85 is connected via a resistor 87 to the line 32 and via aresistor 88 and diode 89 to line 31, so that the transistor acts as aconstant current regulator for the potentiometers 81, 82 and 83. Thethree potentiometers can thus be set independently to adjust the zerosettings for different colours irrespective of the setting of resistor84 which provides an override control. This override control enables theinstrument to operate as a null point indicator.

In a densitometer with a light source, the override control may beprovided by a mechanical iris in the light path. However when theinstrument is used as an on easel color analyser, the controls 81 to 83may be set so that the meter reads null when the light from a referencenegative and its printing filters is received. When the unknown negativeis inserted in place of the reference negative, the overall light levelon one of the colors needs to be adjusted to the reference null levels.In such an instrument, a mechanical iris would only give a limitedamount of control. By use of the circuit shown in FIG. 3, the overallcontrol is provided by resistor 84 which gives a wider range of controlthan a mechanical iris. The controls 81 to 83 provide a memory.

Although the embodiments described have a meter giving a visualindication, the output voltage may be applied to a recorder or may beused as an error signal for process control purposes.

I claim:

1. A radiation sensing device responsive to ultra-violet, visible orinfra-red radiation comprising a photomultiplier tube having an anode, acathode and dynodes with a bleeder resistor chain across the dynodes, anE.I-I.T. supply means applying an E.l-l.T. voltage between the anode andcathode and also across the bleeder resistor chain, said E.l-I.T. supplymeans including a step-up transformer and a rectifier, an oscillatorunit supplying alternating current to the transformer, means responsiveto the anode current of the photomultiplier tube arranged to control theamplitude of the output of the oscillator output unit so that the E.H.T.voltage is decreased as the anode current tends to increase so as tomaintain the anode current substantially constant and indicator meansresponsive to the E.H.T. voltage.

2. A radiation sensing device as claimed in claim 1 wherein said meansresponsive to the anode current of the multiplier tube comprises aresistor in series with the anode and an amplifier responsive to thevoltage across said resistor to provide a dc. supply for the oscillatorunit whereby the oscillator unit output voltage depends on the amplifierinput.

3. A radiation sensing device as claimed in claim 2 wherein saidoscillator unit comprises a multivibrator.

4. A radiation sensing device as claimed in claim 1 wherein saidoscillator unit includes a power output stage coupled to the primarywinding of said transformer.

5. A radiation sensing device as claimed in claim 1 wherein saidindicator means comprises an indicator responsive to the voltage acrossa measuring resistor in a potential divider circuit including the dynodebleeder resistor chain.

6. A densitometer comprising a radiation sensing device as claimed inclaim 1 in combination with a light source and a stabilized power supplyproviding power both for the oscillator unit and said light source.

7. A radiation sensing device responsive to ultra-violet visible orinfra-red radiation comprising:

a. a photomultiplier tube having an anode, a cathode and dynodes with ableeder resistor chain across the dynodes,

b. an E.I-I.T. supply means applying an E.H.T. voltage between the anodeand cathode,

c. said E.H.T. supply means including a step-up transformer and arectifier, an oscillator unit supplying alternating current to thetransformer and means responsive to the anode current of thephotomultiplier tube arranged to control the amplitude of the output ofthe oscillator unit so that the E.H.T. voltage is decreased as the anodecurrent tends to increase,

d. a potential divider circuit including said bleeder resistor chain anda measuring resistor,

e. said E.H.T. supply means applying E.l-I.T. voltage across saidpotential divider circuit,

f. an indicator responsive to the voltage across said measuringresistor,

g. at least two diodes, each with a series resistor, connected toseparate adjustable potential sources, each diode being arranged toby-pass part of the current through said measuring resistor in thepotential divider circuit when the potential across the measuringresistor reaches that of the adjustable potential source to improve thelinearity of the relationship between the logarithm of intensity ofincident radiation and indicated voltage,

h. a further diode in said potential divider circuit between saidmeasuring resistor and the by-pass circuits to correct for changes dueto changes with temperature of said diodes in the by-pass circuits.

8. A radiation sensing device responsive to ultra-violet, visible orinfra-red radiation comprising:

a. a photomultiplier tube having an anode, a cathode and dynodes with ableeder resistor chain across the dynodes,

b. an E.l'-I.T. supply means applying an E.H.T. voltage between theanode and cathode,

c. said E.H.T. supply means including a step-up transformer and arectifier, an oscillator unit supplying alternating current to thetransformer and means responsive to the anode current of thephotomultiplier tube arranged to control the amplitude of the output ofthe oscillator unit so that the E.H.T. voltage is decreased as the anodecurrent tends to increase,

(I. a potential divider circuit including said bleeder resistor chainand a measuring resistor,

e. said E.I-I.T. supply means applying E.l-I.T. voltage across saidpotential divider circuit, and

a differential voltage indicator with an adjustable potential input andresponsive to the difference between the voltage across said measuringresistor and that of said adjustable potential input.

includes an adjustable resistor forming an override control operativeirrespective of the setting of said switch means.

11. A color analyser comprising a radiation sensing device as claimed inclaim 9 in combination with a plurality of color filters selectivelymovable into the radiation path with means for synchronizing theoperation of the switch with the selection of the color filter.

1. A radiation sensing device responsive to ultra-violet, visible orinfra-red radiation comprising a photomultiplier tube having an anode, acathode and dynodes with a bleeder resistor chain across the dynodes, anE.H.T. supply means applying an E.H.T. voltage between the anode andcathode and also across the bleeder resistor chain, said E.H.T. supplymeans including a step-up transformer and a rectifier, an oscillatorunit supplying alternating current to the transformer, means responsiveto the anode current of the photomultiplier tube arranged to control theamplitude of the output of the oscillator output unit so that the E.H.T.voltage is decreased as the anode current tends to increase so as tomaintain the anode current substantially constant and indicator meansresponsive to the E.H.T. voltage.
 2. A radiation sensing device asclaimed in claim 1 wherein said means responsive to the anode current ofthe multiplier tube comprises a resistor in series with the anode and anamplifier responsive to the voltage across said resistor to provide ad.c. supply for the oscillator unit whereby the oscillator unit outputvoltage depends on the amplifier input.
 3. A radiation sensing device asclaimed in claim 2 wherein said oscillator unit comprises amultivibrator.
 4. A radiation sensing device as claimed in claim 1wherein said oscillator unit includes a power output stage coupled tothe primary winding of said transformer.
 5. A radiation sensing deviceas claimed in claim 1 wherein said indicator means comprises anindicator responsive to the voltage across a measuring resistor in apotential divider circuit including the dynode bleeder resistor chain.6. A densitometer comprising a radiation sensing device as claimed inclaim 1 in combination with a light source and a stabilized power supplyproviding power boTh for the oscillator unit and said light source.
 7. Aradiation sensing device responsive to ultra-violet visible or infra-redradiation comprising: a. a photomultiplier tube having an anode, acathode and dynodes with a bleeder resistor chain across the dynodes, b.an E.H.T. supply means applying an E.H.T. voltage between the anode andcathode, c. said E.H.T. supply means including a step-up transformer anda rectifier, an oscillator unit supplying alternating current to thetransformer and means responsive to the anode current of thephotomultiplier tube arranged to control the amplitude of the output ofthe oscillator unit so that the E.H.T. voltage is decreased as the anodecurrent tends to increase, d. a potential divider circuit including saidbleeder resistor chain and a measuring resistor, e. said E.H.T. supplymeans applying E.H.T. voltage across said potential divider circuit, f.an indicator responsive to the voltage across said measuring resistor,g. at least two diodes, each with a series resistor, connected toseparate adjustable potential sources, each diode being arranged toby-pass part of the current through said measuring resistor in thepotential divider circuit when the potential across the measuringresistor reaches that of the adjustable potential source to improve thelinearity of the relationship between the logarithm of intensity ofincident radiation and indicated voltage, h. a further diode in saidpotential divider circuit between said measuring resistor and theby-pass circuits to correct for changes due to changes with temperatureof said diodes in the by-pass circuits.
 8. A radiation sensing deviceresponsive to ultra-violet, visible or infra-red radiation comprising:a. a photomultiplier tube having an anode, a cathode and dynodes with ableeder resistor chain across the dynodes, b. an E.H.T. supply meansapplying an E.H.T. voltage between the anode and cathode, c. said E.H.T.supply means including a step-up transformer and a rectifier, anoscillator unit supplying alternating current to the transformer andmeans responsive to the anode current of the photomultiplier tubearranged to control the amplitude of the output of the oscillator unitso that the E.H.T. voltage is decreased as the anode current tends toincrease, d. a potential divider circuit including said bleeder resistorchain and a measuring resistor, e. said E.H.T. supply means applyingE.H.T. voltage across said potential divider circuit, and f. adifferential voltage indicator with an adjustable potential input andresponsive to the difference between the voltage across said measuringresistor and that of said adjustable potential input.
 9. A radiationsensing device as claimed in claim 8 wherein there are provided aplurality of adjustable potential sources with switch means forconnecting any selected one of said adjustable potential sources to saidadjustable potential input of the differential voltage measuringindicator.
 10. A radiation sensing device as claimed in claim 9 whereinsaid plurality of adjustable potential sources comprises a plurality ofadjustable potentiometers connected in shunt in a constant currentcircuit, which constant current circuit also includes an adjustableresistor forming an override control operative irrespective of thesetting of said switch means.
 11. A color analyser comprising aradiation sensing device as claimed in claim 9 in combination with aplurality of color filters selectively movable into the radiation pathwith means for synchronizing the operation of the switch with theselection of the color filter.